The present invention relates to electric field poling of ferroelectric materials, particularly to a method of inducing periodic poling in surface regions of samples of ferroelectric materials, and to optical waveguide devices comprising ferroelectric materials having periodic poling in surface regions.
Domain engineering in ferroelectric materials is becoming an increasingly important topic as the demand grows from the optical telecommunication market for new integrated optical devices. The technique involves engineering of a material so that its nonlinear coefficient (nonlinearity) is periodically reversed to form a grating in a chosen direction. The material can then be used for “quasi-phasematching” of nonlinear optical interactions, which can vastly increase the interaction efficiency over that achievable in non-engineered material. The engineering process is commonly known as “periodic poling”, and material which has been treated in this way is described as “periodically poled”. Periodically poled lithium niobate and lithium tantalate crystals are now commercially available products for use as efficient frequency converters or as nonlinear media for optical parametric processes.
Numerous techniques exist which have been applied for periodic poling, most of them designed to create periodic 180° domain structures with a specified mark-to-space ratio extending throughout the full depth of the material. A particularly common technique is that of electric field poling, whereby an electric field is applied across a sample of ferroelectric material. This causes inversion of crystal domains in the material, which reverses the polarity and hence the nonlinearity. The periodicity is achieved by applying a metal mask/electrode structure corresponding to the desired pattern of poling to a surface of the sample before applying the electric field. It is necessary to make very accurate calculations of the electric charge flowing during the poling process, as this governs the level of domain inversion achieved, and hence the quality of the resulting poled sample. Too much charge results in sideways growth of the inverted regions, which can affect the mark-to-space ratio achieved. Also, high electric fields are required for bulk domain reversal in ferroelectrics such as lithium niobate and lithium tantalate. Inherent nonuniformities and defects present in commercially available and doped (in the case of waveguide fabrication) materials can restrict the applicability of standard bulk electric field poling techniques of this type to the engineering of large periods of order >15 μm. For first order nonlinear interactions such as frequency doubling, however, shorter periods are required (of the order of a few micrometres). These are difficult to fabricate routinely and in many cases the material has to be polished down to thickness of order 150 μm or less [1, 2].
Two approaches aimed at overcoming this apparent limit on domain period have recently been reported. A first approach, referred to as controlled spontaneous backswitching, has been applied to bulk samples of lithium niobate of typical thickness 500 μm, to create periods of 4 μm [3], and more recently 2.6 μm [4]. The applied electric field is rapidly modulated, which exploits a natural tendency of recently-inverted domains at the edge of inverted areas to return to their original orientation (backswitching). Thus, the width of the inverted area reduces, altering the mark-to-space ratio of the poling. Finally, a stabilisation voltage is applied to prevent further backswitching. Uniform short period poling can be engineered in this way, but the complicated variation of the electric field requires complex voltage control and detailed calculations.
A second approach, applied to MgO:LiNbO3 which has the benefit of improved resistance to photorefractive damage, utilises multiple short voltage pulses and has been used to generate a period of 2.2 μm at a depth of 1.5 μm [5]. The resulting sample was used in conjunction with a waveguide geometry and produced a high nonlinear optical conversion efficiency. Once again, however, the use of multiple current pulses requires complex control.
A further feature of the standard bulk electric field poling and the controlled spontaneous backswitching techniques is that they aim to produce poling through the whole depth of the sample. However, when the nonlinear optical application involves planar or channel waveguides, the requirement for deep domain inversion is no longer important so long as there is a good overlap between the guided optical fields and the inverted domain regions. Typical depth dimensions in the case of waveguides (and hence required depth of domain inversion) lie in the region of only about 2 μm to 10 μm. Hence it is not necessary to rely on poling techniques which aim to achieve uniform high quality domain engineering at depths exceeding this. One technique which is addressed to creating shallow poling is based on Li2O out-diffusion [6]. This can cause superficial domain inversion, but cannot achieve deep enough domains for sufficient overlap with the guided optical fields used in waveguides. Furthermore, it requires high temperature annealing processes.
Thus, it would be of great benefit to provide a simple poling process capable of accurately engineering short domain periods, including submicron periods, which do not need to extend throughout the thickness of a sample, to overcome the complexities and drawbacks of known processes.